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E46 (1999 - 2006) The fourth generation 3 Series (E46 chassis) was introduced in 1999 and set the standard for engineering and performance during it's years of production including being named to Car & Driver's 10 best list every one of those years! ! -- View the E46 Wiki

I'm sure there's a thread on this somewhere, feel free to redirect me...

2003 330xi with automatic transmission. Just replaced a failed expansion tank and ATF thermostat. I followed all the instructions I could find here and elsewhere on how to properly bleed the coolant system. The car runs fine, heater works fine, temp guage stays where it should, coolant level (visual check) remains full. But, that freakin' coolant level sensor light keeps coming back on. It stays off until I put the car in gear.

I'm assuming I've got air trapped somewhere, but as I say, the car is not overheating, the coolant level in the expansion tank is appropriate, and I've followed all the tips for proper bleeding. With the car running, the upper radiator does get hot, as you would expect, so there is heat exchange going on.

Is it possible that the area in the expansion tank where the sensor sits is not getting coolant? Anyone else have this issue?

I think either your level sensor is bad, or the e tank you got has a faulty target/pickup for the sensor. I've heard of this before.

If you still have your old tank you could pull the sensor out of the new one and see if you can insert it into the old tank while unde the car, then pull the float stick all the way up in the old tank and see if the light goes out then. Would confirm bad target in new tank.

Ah, that would be clever, but the new tank didn't come with a sensor... I had to pull it from the old tank. I also thought the sensor might be bad and ordered a new one along with the ATF thermostat. Unfortunately, I order the less expensive sensor and its plastic housing shattered while I was installing it... so it was back to the old one.

I plan on overhauling the entire cooling system in the spring (per advice found here), and I'll probably replace the sensor at that time as well... it's just that staring at that coolant level light on the dashboard gives me so much anxiety knowing how much damage overheating can cause.

Ah, that would be clever, but the new tank didn't come with a sensor... I had to pull it from the old tank. I also thought the sensor might be bad and ordered a new one along with the ATF thermostat. Unfortunately, I order the less expensive sensor and its plastic housing shattered while I was installing it... so it was back to the old one.

I plan on overhauling the entire cooling system in the spring (per advice found here), and I'll probably replace the sensor at that time as well... it's just that staring at that coolant level light on the dashboard gives me so much anxiety knowing how much damage overheating can cause.

You can buy a direct sensor to the coolant - I know there's a thing called Coolant Snitch available on the ZHP registry site. I'm also planning to replace my cooling system just to be safe but the only non-OEM addition will be an accurate coolant temp. sensor.
Much more direct and will tell you what your temp. is sooner than that little needle.

Ah, that would be clever, but the new tank didn't come with a sensor... I had to pull it from the old tank. I also thought the sensor might be bad and ordered a new one along with the ATF thermostat. Unfortunately, I order the less expensive sensor and its plastic housing shattered while I was installing it... so it was back to the old one.

I plan on overhauling the entire cooling system in the spring (per advice found here), and I'll probably replace the sensor at that time as well... it's just that staring at that coolant level light on the dashboard gives me so much anxiety knowing how much damage overheating can cause.

The sensor sticks up into a recess in the outside of tank, so you can remove it on the car without spilling fluid. So the test I suggested will not spill any coolant.

I think either your level sensor is bad, or the e tank you got has a faulty target/pickup for the sensor. I've heard of this before.

If you still have your old tank you could pull the sensor out of the new one and see if you can insert it into the old tank while unde the car, then pull the float stick all the way up in the old tank and see if the light goes out then. Would confirm bad target in new tank.

oh, that's how the sensor works?
i have a very similar problem like OP's.
after changing expansion tank 3 years ago (wrong decision to pick the Uro), which came with a sensor, the low coolant light keeps coming on.

recently, i got under the car, pulled the connector from the sensor and plugged it into my old sensor and stuck the metal part of the old sensor into a bottle of water, but light still didn't go off.
i was assuming that the sensor works by detecting whether it's soaked into water.

if indeed it works by detecting the floating stick's position, i should probably give it a try by swapping the old sensor in, since you also said that this kind of operation won't let any coolant out, so i wouldn't need to bleed the system.

i did some more research on this and believe i have a better understanding now.
it's a simple magnetic float switch (or Reed switch/sensor).
coolant doesn't need to physically touch the sensor, there's a permanent magnet (possibly attached with the float stick), which keeps the two metal contacts in the sensor from contacting each other.
but when the coolant is too low, the magnet will drop to a point that is too far from the glass tube of the sensor where the two contacts reside; consequently the two metal contacts will go back to its natural position and touch each other to complete the circuit and turns on the light.
so if the permanent magnet near the sensor is indeed attached to the float stick, what ahull said makes perfect sense.

this is the thread i found about it: http://forums.bimmerforums.com/forum...-it-work/page2
on page 1 of the thread, they were talking about a comparatively more complicated theory of operation, which was then corrected by post #29 on page 2.
the person who explained the incorrect theory in page 1 also acknowledged that in post #30.
post #32 also briefly explains how it works.

Thanks so much! This is great knowledge to have going forward... great research!

I'm afraid my situation got a bit more mysterious... the coolant level sensor is now intermittent. It's off at idle, comes on as I start driving, goes off when I get up to cruising speed, comes back on and goes back off periodically after that... I'm thinking there must be air trapped in there somewhere.

Thanks so much! This is great knowledge to have going forward... great research!

I'm afraid my situation got a bit more mysterious... the coolant level sensor is now intermittent. It's off at idle, comes on as I start driving, goes off when I get up to cruising speed, comes back on and goes back off periodically after that... I'm thinking there must be air trapped in there somewhere.

thanks again!

if the system is not bled properly, you can always follow the bleeding procedures.

i put my original coolant level sensor back in and the dashlight finally turns off! (previously, i had plugged the connector to this original sensor but without putting it back to the ET, nor near any sort of magnet, and the dashlight was still on)

i've tested both sensors with a magnet and a multimeter and found that when a magnet is near the probe of the sensor, the sensor's circuit is actually closed and when no magnet is near, the circuit is in fact open.
this is opposite to what i described in post #10 above....

by carefully going through the rest of the other thread that i mentioned, things seem to be consistent with my new understanding:

- coolant level sensor is a reed sensor/switch
- a magnet is attached to the float stick
- the magnet is near the sensor probe (glass tube on the sensor) when float stick is at the desired level
- sensor circuit is not the same circuit to power on the low coolant dashlight
- magnet present next to sensor probe => sensor circuit closed => low coolant dashlight off
- magnet away from sensor => sensor circuit open => low coolant dashlight on
- idea for not having the sensor circuit to server as the power source powering the low coolant dashlight:
in case of circuit (sensor circuit) failure or magnet detached from float stick, etc., low coolant dashlight will still light up to indicate malfunction to user
- but to test the actual circuit that powers on the low coolant dashlight, we can always do a dashlight test

what's left as a doubt is that, why both sensors behave the same when i test them with a magnet and a multimeter while one functions properly in the ET, the other doesn't
only reason i can think of is that the one came with the Uro brand ET is not built with high quality and the probe is not at the correct position.